Discriminating Disease Flare From Infection in Febrile Patients With Systemic Lupus Erythematosus in a Safety-Net Hospital System: A Multicenter Study.

Objective: The objective of this study was to assess clinical laboratory parameters that distinguish between disease flare and infection in febrile patients with systemic lupus erythematosus (SLE) at safety-net hospitals in Los Angeles.

Methods: We reviewed electronic medical records of patients admitted from August 1, 2016, through July 31, 2019, categorizing them as disease flare, bacterial infection (culture positive), culture-negative infection, and both flare and infection. Laboratory parameters collected within 48 hours of admission (complete blood cell count with differential, liver function panel, erythrocyte sedimentation rate [ESR], C-reactive protein [CRP], C3, C4, lactate, procalcitonin, and ferritin) were analyzed.

Results: Several laboratory parameters significantly distinguished febrile patients with disease flare from those with infection. An optimized multivariable logistic regression model revealed that an elevated ESR:CRP ratio (>1.17), low white blood cell (WBC) count (<6.25 × 10⁹/L), low absolute neutrophil count (<5.55 × 10⁹/L), and low CRP (<113 mg/L), C3 (<44.5 mg/dL), and C4 (<13.5 mg/dL) levels helped discriminate disease flare from culture-positive infection. These laboratory parameters yielded areas under the receiving operating characteristic curve of 0.87 (95% confidence interval [CI] 0.76-0.97) for flare versus culture-positive infection and 0.94 (95% CI 0.88-1.00) for flare versus culture-negative infection. These optimized models, using multiple laboratory parameters, significantly outperformed the ESR:CRP ratio alone (P < 0.02) in discriminating flare from infection.

Conclusion: The ESR:CRP ratio plus C3 and C4 levels, WBC count, neutrophil count, and monocyte count discriminate flare from either culture-positive or culture-negative infection in febrile patients with SLE. Our findings warrant prospective validation.